When producing elastomeric compositions for use in rubber articles, such as tires, power belts, and the like, it is desirable that these elastomeric compositions are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, modulus, tangent delta (tan δ), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties will have reduced hysteresis, better rolling resistance, snow and ice traction, wet traction, and improved fuel economy for vehicles equipped with such tires.
Dispersion of silica filler has been a major concern in tire processing because polar silanol groups on the surface of silica particles tend to self-associate and reagglomeration of silica particles can occur after compounding, leading to poor silica dispersion and a high compound viscosity. The strong silica filler network results in a rigid uncured compound that is difficult to process in extrusion and forming operations. Various silica coupling agents and silica shielding agents have been employed to address these problems. For example, bifunctional silica coupling agents having a moiety (e.g., a silyl group) reactive with the silica surface and a moiety (e.g., sulfur) that binds to the elastomer are well known and include mercaptosilanes, blocked mercaptosilanes, bis(trialkoxysilylorgano) polysulfides, and the like. These commonly-used sulfur-containing bifunctional silica coupling agents can offer excellent coupling between rubber and silica; however, there are disadvantages to their use. For example, the high chemical reactivity of the —SH functions of the mercaptosilanes (e.g., γ-mercaptoalkyltrialkoxysilanes, 3-thiocyanatopropyl trimethoxysilane, and the like) with organic polymers can lead to unacceptably high viscosities during processing and to premature curing (scorch), making compounding and processing more difficult. Rubber compounds employing bis(trialkoxysilylorgano) polysulfides often have mixing temperature limitations to avoid thermal degradation of the agents and premature increase in the viscosity of the rubber mixture. In general, when compared with carbon black-filled compositions, a silica coupling agent is needed to obtain tread compounds having good silica dispersion.
Several approaches to improving dispersion of silica in rubber compounds have been directed to reducing or replacing the use of such sulfur-containing silica coupling agents by employing silica dispersing aids, such as silica shielding agents that chemically react with, and/or physically shield the surface silanol groups on the silica particles but are not reactive with the elastomer. Although these agents can improve processability of the compound, in some cases they may not provide adequate physical properties to the rubber compound when used alone because they do not chemically bind to the rubber backbone.
Recently, non-sulfur alkoxysilane coupling agents, such as citraconimido-alkoxysilane coupling agents, have been described in U.S. Pat. Nos. 6,878,768 and 7,238,740. However, a major problem with using these silica/rubber coupling agents is that they are only effective when combined with an isoprene elastomer, and are ineffective in the absence of such an elastomer. That is, when the diene elastomer consists essentially of another synthetic elastomer such as styrene-butadiene rubber (SBR), conventionally used in treads for tires, the excessive reactivity, increase in viscosity and insufficient coupling performance, are still a major problem. Moreover, it was disclosed that, despite a similar chemistry, a similar maleimido-triethoxysilane coupling agent produced a greatly increased viscosity in the uncured isoprene rubber compared with the citraconimido-trialkoxysilane coupling agent.
Therefore, there is a need for rubber compositions that do not have the above-described limitations, regardless of the diene rubber employed, that can make use of non-sulfur silica coupling agents that can bond to the rubber backbone.